Note: Descriptions are shown in the official language in which they were submitted.
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IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
Liquid Crystal Displays with Polarized Infrared Illumination
FIELD OF THE INVENTION
[0001] The present disclosure relates to infrared readable liquid crystal
displays, and more
specifically, to illuminating liquid crystal displays with polarized infrared
light to extend
readability beyond the usable contrast range of the liquid crystal display
polarizers to longer
wavelengths.
BACKGROUND OF THE INVENTION
[0002] Modern liquid crystal displays (LCDs) can comprise millions of
individual pixels,
which in an LCD is a thin multilayered structure of many components. Pixels in
an LCD are
illuminated using an unpolarized light source, such as light emitting diodes
(LEDs).
[0003] Most LCD pixels are based on two functional principles: 1) an
electrically
controllable liquid crystal layer between transparent substrates changes the
polarization state
of the light passing through it based on applied electrical signals and 2) one
or more
polarizers and optional additional optical films transform a difference of
polarization states
into visible bright and dark contrast regions. Together the polarizer(s) and
the liquid crystal
layer form an electrically controllable light valve, which lets a portion of
the light pass
through depending on the electrical stimulus.
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[0004] Since most LCDs are flat or slightly curved and may have a substantial
size,
generally film polarizers are required. Such films are adhered to one or more
surfaces of the
display and cover substantially the entire image area. Film polarizers
typically comprise two
layers of a transparent isotropic polymer such as cellulose Tr-Acetate (TAC),
sandwiching a
stretched Poly Vinyl-Alcohol (PVA) layer containing anisotropic chromophores,
which align
due to the stretching of the PVA and provide the polarization effect.
[0005] The chromophores are typically iodine, which is present in the PVA as
13- and Is-
PVA complexes. Alternatively, the PVA layer may be impregnated with different
anisotropic
dye molecules that together cover the visible range of the spectrum. Common to
either the
iodine method or the dye method is that the range of usable contrast is
limited to the visible
range of the spectrum as the PVA-iodine complexes and the typically used
anisotropic dyes
are transparent in the near infrared. Hence, when viewing normal LCD's with IR
sensitive
cameras or night vision equipment, they do not show an image when operated
under non-
visible IR-illumination only.
[0006] It is sometimes desirable or even necessary that LCDs can be viewed
using invisible
infrared light. For example, it may be necessary to read an LCD with night
vision goggles in
the absence of any visible light, or it may be necessary to read LCDs with
infrared cameras.
[0007] Common to most outdoor optical equipment is that it selectively uses
vertically
polarized light to eliminate glare from glancing reflections of smooth or wet
surfaces. This is
achieved by adding polarization elements to the front of the camera lenses.
Such polarizing
elements can be selected so that they can reject visible and/or IR light with
an undesired
polarization state.
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[0008] While it is possible to add additional types of dye molecules that are
absorbing at
longer infrared wavelengths to the chromophore layer of a polarizer to extend
the polarizer's
usable contrast range, this is undesirable as such polarizers necessarily have
a lower
transmission in the visible range, leading to darker displays. This is because
adding more dye
molecules reduces transmission, and most dyes have higher order absorption
bands. For
example, a dye that would absorb at 900nm in the infrared range would likely
also absorb at
around 450nm, which is in the visible range. A higher absorption (or lower
transmission) is a
problem especially for reflective displays or battery-operated displays where
the lower
visible transmission cannot be compensated for with a stronger light source.
[0009] Alternate types of polarizers such as wire grid polarizers, cholesteric
film
polarizers, and multilayer birefringent stack polarizers function on the
principle of
transmitting one polarization state while reflecting the other. These types of
polarizers are an
option for the rear side of a display, if backed by a suitable absorber for
the transmitted
polarization. Such polarizers typically have good contrast in the near
infrared as they are not
based on absorption of dyes. For example, a common 3M DBEF polarizer or a
Nagase WGF
works well to 850nm. Such devices can be used instead of absorptive rear
polarizers in an
LCD. For example, an IR wire grid polarizer can be placed behind a display as
the rear
polarizer. However, it is undesirable to use such polarizers on the front of a
display as they
have a metallic, mirrorlike appearance due to the specular reflection of about
half the
incoming light. If they are used in front of the display, they will have to be
hidden under an
additional absorptive polarizer that will remove the reflected portion of the
light in the visible
range, while taking appropriate steps that such specular reflection of
infrared light will not
interfere with reading the display with infrared equipment.
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[0010] Thus, it would be desirable to extend the usable contrast range of LCDs
to longer
wavelength, and to make them readable by infrared equipment without the side-
effects of
lower brightness in the visible range, mirrorlike appearance, or the need of
additional
polarizers that prevent the mirrorlike appearance.
[0011] BRIEF SUMMARY OF THE INVENTION
[0012] For purposes of summarizing the invention, certain aspects, advantages,
and novel
features of the invention have been described herein. It is to be understood
that not
necessarily all such advantages may be achieved in accordance with any one
particular
embodiment of the invention. Thus, the invention may be embodied or carried
out in a
manner that achieves or optimizes one advantage or group of advantages as
taught herein
without necessarily achieving other advantages as may be taught or suggested
herein.
[0013] Many liquid crystal displays have an integrated light source, either as
a backlight or
a front light. The front light or backlight typically is composed of the
actual light source and
an 'integrator', which distributes the light evenly across the display. The
light sources are
typically CCFL tubes, electroluminescent films, or, most suitably, light
emitting diodes, such
as either solid state LEDs or organic LEDs. An integrator may be a light
cavity or a
transparent lightguide that distributes the light evenly across the display
surface.
[0014] Since LEDs are available, which emit light in the required range of
infrared
wavelength, such LEDs can replace or be added to visible light LEDs to create
an infrared
illuminator. This invention and disclosure comprise such an illuminator or
integrator, which
is a lightguide that is designed to maintain the polarization state of the
light. The display can
use a regular, visible-light, front polarizer and hence does not suffer from a
brightness
reduction caused by an IR-capable polarizer.
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[0015] A polarizer in front of an LCD display serves two functions: 1) it
polarizes
incoming light and 2) it analyzes, or turns into contrast, the polarization
state of the light
coming out of the display. If the observer uses polarized light, for example
with polarized
sunglasses or, more suitably, an optical device, such as a camera or night
vision equipment,
the second function is already provided by this equipment. Hence, to extend
the usable
wavelength range of an LCD display into the IR range, either both functions
need to be
extended into the IR range, or if it can be assured that any relevant IR
equipment has its own
polarizer, it is sufficient to only extend polarizing the incoming light into
the IR range.
[0016] Visible range polarizers are isotropic and transparent in the infrared
range used by
night vision equipment or infrared cameras, specifically in the range of 700
to 1100nm.
Hence, if a display is illuminated with polarized IR light it will remain
readable to IR
detection equipment that uses an IR-polarizer for glare reduction.
[0017] If a display has an infrared-capable, rear polarizer, illuminating it
with a polarized
IR source will create contrast even without a polarizer on the optical
equipment as light will
be reflected out through the display, depending on the polarization state
after the first path
through the display.
[0018] One exemplary embodiment of this invention and disclosure is an
infrared
transmissive LCD display with a backlight comprising a lightguide and LED edge
illumination. In addition to visible-light LEDs, IR-emitting LEDs are added to
the edge of the
lightguide. An infrared polarizer, such as a wire grid polarizer, is placed
between the infrared
and optionally the visible LEDs and the lightguide edge to provide a suitable
polarization
state. The light guide is designed to maintain that polarization.
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[0019] In visible light operation polarized or unpolarized visible light is
guided through the
lightguide into the display and passes through the display from the back to
the front. When
exiting the lightguide, the light first encounters a polarizer that polarizes
it into the desired
state. It then passes through the liquid crystal layer, which can change the
polarization of the
light depending on the desired state of a pixel. Finally, it passes through
the front polarizer,
which acts as the analyzer and thus creates a brightness contrast, visible to
the naked eye.
[0020] In infrared operation, the IR LEDs emit light that is polarized when
passing through
the IR polarizer. The polarized IR light passes through the lightguide and is
directed towards
the LCD. The polarizer in the LCD appears transparent to the IR light. The
liquid crystal
layer changes the polarization state if needed, based on the state of the
pixel. The front
polarizer appears transparent to the IR light and hence does not act as an
analyzer.
[0021] As a result, some areas of the display emit light in one polarization
state whereas
other areas emit light in another polarization state. Infrared sensitive
optical equipment such
as infrared cameras or night vision goggles with an IR antiglare polarizer can
detect the
image as their polarizer acts as the analyzer, turning the polarization
differences into a
brightness contrast for the sensor element. Human eyes cannot detect
differences in the
polarization state of the light, but they can detect differences in
brightness. The function of
the analyzer is to turn differences in polarization states into differences in
brightness by
letting through one polarization state while absorbing or reflecting the
other. This
embodiment may also include further optical films, such as retardation films,
compensation
films, or other light management films that optimize the performance of the
device.
[0022] Another exemplary embodiment of this invention and disclosure is a
reflective LCD
display with polarized IR illumination. In visible light operation, ambient
light or light from
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visible light LEDs in the front light travels through a front light that
leaves it unchanged, and
then to the front polarizer where the light is polarized. In an alternate
embodiment, the
ambient visible light first encounters a visible light (absorptive) polarizer
that imparts a
specific polarization state, which remains unchanged when traveling through
the front light
guide. The polarized light then enters the liquid crystal layer where its
polarization state may
be changed depending on the electrical signals applied. It may then pass
through an optional
polarizer before being reflected by a mirror, or it may be reflected by a
polarizing mirror
such as a reflective polarizer, backed with an absorber.
[0023] On the return path, the light again passes through the liquid crystal
layer and
through the front polarizer, which acts as the analyzer, turning differences
in polarization
state into a brightness contrast. Finally, the light passes through the front
light, which appears
mostly transparent to the light. In an alternate embodiment, the light first
encounters the front
light guide where its polarization state remains unchanged, before being
analyzed in the front
polarizer.
[0024] In IR light operation, IR light from the IR LEDs is coupled into the
front light
lightguide via an IR polarizer. The polarized IR light passes through the
lightguide, which
may be positioned in front of the front polarizer or between the front
polarizer and the LCD
without change in polarization state. The lightguide sends the light through
the LCD, where
the polarization state of the light can be changed according to the signals
applied. The light is
selectively reflected at only one polarization by a reflective polarizer or
polarizer-mirror
combination behind the display, while the other polarization is absorbed.
[0025] The reflected light returns through the liquid crystal layer, where
further
polarization adjustment may happen. Upon exiting the LCD layer, the light
encounters the
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front polarizer and front light, which both appear transparent to the IR
light. As a result,
some areas of the display appear to emit IR light, while others do not.
Infrared sensitive
optical equipment will detect different brightness levels depending on the
liquid crystal state
even without an IR antiglare polarizing element. This embodiment may also
include further
optical films such as retardation films, compensation films, diffuser films
and other light
management films that optimize the performance of the device.
[0026] Another exemplary embodiment of this invention and disclosure is a
reflective
liquid crystal display for a digital license plate application comprising a
reflective liquid
crystal display with a front light, which includes polarized IR illumination.
License plate
recognition systems operate at specific infrared wavelengths, such as 750nm,
850nm, 870nm
and others. The reflective LCD may be a bi-stable or multi-stable LCD due to
the low power
requirements of such displays compared to displays requiring constant
updating. One such
bistable LCD type may be a Memory-in-Pixel LCD, another may be a bistable
nematic LCD
known as `Binem' or a bistable nematic display known as `ZBD'.
[0027] The LCD may work on a single polarizer basis or have a reflective rear
polarizer,
such as a multilayer polymer stack available from 3MTm known as DBEF, a wire
grid
polarizer such as a Nagase WGF, or similar, which have usable contrast from
about 380nm to
greater than 850nm. The front lightguide may be located in front of or behind
the front
polarizer. The front lightguide is illuminated from the edge with optional
white light LEDs
for night visibility and with a plurality of IR LEDs selected for a desired
wavelength or
multiple desired wavelengths depending on the requirements of the location
where such a
license plate is issued. For example, a display may be fitted with several
750nm and several
850nm LEDs if that matches the requirement. Other combinations are possible as
well.
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[0028] A narrow polarizer strip of a polarizer with good polarization
efficiency at the
desired wavelengths is placed between the IR LEDs and the lightguide. Such
polarizer may
be a dye type polarizer with dye selected for infrared operation only and it
may not have
good transmission or polarization efficiency in the visible spectrum as no
visible light is
required to pass through it. Another suitable type of polarizer may be a wire
grid type
polarizer or multi-layer stack polarizer as such polarizers are simpler and
easier to produce at
a lower cost than wire grip polarizers for the visible range.
[0029] A polarization preserving lightguide may be made from transparent
polymers, glass,
or a combination of different transparent materials and may be coated with
materials with
different refractive indices. The light travels from the light source through
the lightguide due
to total internal reflection. Additional features such as certain shapes of
alternating materials
or certain surface structures, such as dimples or prisms, cause the light to
be sent towards the
display, but not to the opposite surface.
[0030] The light passes through the liquid crystal layer where its
polarization may get
changed according to the liquid crystal alignment before encountering a
selective reflection
in the rear reflective polarizer. In dark areas the light has a polarization
state that passes
through the reflective polarizer and gets absorbed in a black layer placed
behind the display
assembly. In bright areas, the light gets reflected back through the display,
where further
polarization changes may happen. The front polarizer appears transparent to
infrared light.
[0031] These structures in the front light are designed to allow at least a
portion of the light
being reflected by the display to pass through to the front surface. A license
plate recognition
system with an optional anti-glare IR polarizer on the lens operating on any
of the
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wavelengths that is provided by the IR LEDs will now see different regions of
the display as
bright or dark, depending on the local polarization state of the light.
[0032] Accordingly, one or more embodiments of the present invention overcomes
one or
more of the shortcomings of the known prior art.
[0033] For example, in one embodiment, an infrared light readable liquid
crystal display
system comprises a liquid crystal display comprising a liquid crystal display
cell comprising:
a liquid crystal layer to control the polarization state of visible light and
infrared light, a front
substrate, a rear substrate, and wherein the liquid crystal layer is located
between the front
substrate and the rear substrate; a visible light front polarizer, wherein the
visible light front
polarizer is transparent to infrared light; and a reflective rear polarizer to
polarize visible light
and infrared light; and an illumination unit comprising a plurality of light
sources, wherein at
least one of the plurality of light sources emits infrared light.
[0034] In this embodiment, the infrared light readable liquid crystal display
system can
further comprise: wherein the liquid crystal display is an infrared
transmissive liquid crystal
display, and wherein the illumination unit is a backlight; wherein the
backlight further
comprises an absorber to absorb extra light; wherein the infrared transmissive
liquid crystal
display further comprises a visibly opaque layer transparent to infrared light
and the
backlight further comprises a reflector, wherein the reflector reflects
infrared light; wherein
the backlight further comprises a polarization conserving lightguide, and an
infrared
polarizer, wherein the infrared polarizer is located between the plurality of
light sources and
the polarization conserving lightguide; wherein the visibly opaque layer is
transparent to
infrared light, and wherein the visibly opaque layer appears black in visible
light; wherein the
visibly opaque layer is transparent to infrared light, and wherein the visibly
opaque layer is
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non-black in visible light; wherein the plurality of light sources comprises
at least one
infrared emitting light source and at least one visible light emitting light
source; wherein the
liquid crystal display is a reflective liquid crystal display, and wherein the
illumination unit is
a front light; wherein the front light is located between the visible light
front polarizer and the
liquid crystal display cell; wherein the front light is in front of the
visible light front polarizer
from the perspective of an observer; wherein the front light further comprises
a polarization
conserving lightguide and an infrared capable polarizer, and wherein the
infrared capable
polarizer is located between the illumination unit and the polarization
conserving lightguide;
wherein the plurality of light sources comprises at least one infrared
emitting light source and
at least one visible light emitting light source.
[0035] In another example embodiment, an infrared light readable liquid
crystal display
system for an electronic license plate comprises a liquid crystal display
comprising a liquid
crystal display cell comprising a liquid crystal layer to control the
polarization state of visible
light and infrared light, a front substrate, a rear substrate, and wherein the
liquid crystal layer
is located between the front substrate and the rear substrate; a visible light
front polarizer,
wherein the visible light front polarizer is transparent to infrared light and
a reflective rear
polarizer to polarize visible light and infrared light; an illumination unit
comprising a
plurality of light sources wherein at least one of the plurality of light
sources emits infrared
light; a plurality of light sensors wherein at least one of the plurality of
light sensors is
sensitive to infrared light; and an electronic circuit capable of driving the
at least one of the
plurality of light sources that emits infrared light.
[0036] In this embodiment, the infrared light readable liquid crystal display
system for an
electronic license plate further comprising a microcontroller to receive an
input from the at
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least one of the plurality of light sensors sensitive to infrared light, and
wherein based on the
input the microcontroller controls the at least one of the plurality of light
sources that emits
infrared light; further comprising a separate circuit to receive an input from
the at least one of
the plurality of light sensors sensitive to infrared light, and wherein based
on the input the
separate circuit controls the at least one of the plurality of light sources
that emits infrared
light; wherein the plurality of light sources comprises at least one infrared
emitting light
source and at least one visible light emitting light source; and wherein the
liquid crystal
display maintains a stable visible image without being refreshed more than
once per second.
[0037] In another example embodiment, a method of operating an infrared light
readable
liquid crystal display system comprises providing a liquid crystal display
comprising a liquid
crystal display cell comprising a liquid crystal layer to control the
polarization state of visible
light and infrared light, a front substrate, a rear substrate, and wherein the
liquid crystal layer
is located between the front substrate and the rear substrate; a visible light
front polarizer,
wherein the visible light front polarizer is transparent to infrared light,
and a reflective rear
polarizer to polarize visible light and infrared light; providing an
illumination unit
comprising a plurality of light sources, wherein at least one of the plurality
of light sources
emits infrared light; and controlling the plurality of light sources based on
an external
stimulus.
[0038] In another example embodiment, a method of operating an infrared light
readable
liquid crystal display system for an electronic license plate comprises
providing a liquid
crystal display comprising a liquid crystal display cell comprising a liquid
crystal layer to
control the polarization state of visible light and infrared light, a front
substrate, a rear
substrate, and wherein the liquid crystal layer is located between the front
substrate and the
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rear substrate; a visible light front polarizer, wherein the visible light
front polarizer is
transparent to infrared light; and a reflective rear polarizer to polarize
visible light and
infrared light; providing an illumination unit comprising a plurality of light
sources, wherein
at least one of the plurality of light sources emits infrared light; providing
a plurality of light
sensors wherein at least one of the plurality of light sensors is sensitive to
infrared light;
providing an electronic circuit capable of driving the at least one light
source which emits
infrared light; and controlling the plurality of light sources with the
electronic circuit based
on illumination conditions from the plurality of light sensors.
[0039] Other objects, features, and advantages of the present invention will
become
apparent upon consideration of the following detailed description and the
accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] Figure 1 shows a typical prior art backlight as used with transmissive
or
transflective displays.
[0041] Figure 2 shows a typical prior art front light as used in conjunction
with reflective
displays.
[0042] Figure 3 shows a liquid crystal display with a backlight according to
U.S. Patent
No. 9,190,004.
[0043] Figure 4 shows prior art in accordance with U.S. Patent No. 9,229,268,
which is an
improvement of U.S. Patent No. 9,190,004 as it eliminates the undesirable
mirror-like
appearance.
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[0044] Figure 5A shows one example embodiment of the invention consisting of
an
infrared transmissive LCD display with a reflective rear polarizer and a
backlight backed by
an absorber.
[0045] Figure 5B shows one example embodiment of the invention consisting of
an
infrared transmissive LCD display with a reflective rear polarizer and a
backlight backed by
a mirror.
[0046] Figure 6 shows one example embodiment of the invention consisting of an
infrared
transmissive LCD display with polarization conserving backlight and absorptive
visible light
rear polarizer.
[0047] Figure 7 shows one example embodiment of the invention consisting of a
reflective
LCD display with non-polarized IR front light, reflective rear polarizer, and
IR analyzer on
the camera.
[0048] Figure 8 shows one example embodiment of the invention consisting of a
reflective
LCD display with polarization conserving front light and reflective rear
polarizer.
[0049] Figure 9 shows one example embodiment of the invention consisting of a
digital
license plate display with reflective LCD, front light and combined IR/visible
light
illumination.
[0050] Figure 10 shows one example embodiment of the invention consisting of a
display
system in the form of a block diagram.
[0051] Figure 11 shows one example embodiment of the invention consisting of
an
alternative layout of display system in the form of a block diagram.
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[0052] Figure 12 shows an example circuit consisting of an IR sensing board
and LED
driving board that can be used to detect light and drive the IR LEDs without
involvement of a
microcontroller.
DETAILED DESCRIPTION OF THE INVENTION
[0053] The following is a detailed description of embodiments to illustrate
the principles of
the invention. The embodiments are provided to illustrate aspects of the
invention, but the
invention is not limited to any embodiment. The scope of the invention
encompasses
numerous alternatives, modifications, and equivalents. The scope of the
invention is limited
only by the claims.
[0054] While numerous specific details are set forth in the following
description to provide
a thorough understanding of the invention, the invention may be practiced
according to the
claims without some or all of these specific details.
[0055] Various embodiments will be described in detail with reference to the
accompanying drawings. Wherever possible, the same reference numbers are used
throughout the drawings to refer to the same or like parts. References made to
particular
examples and implementations are for illustrative purposes and are not
intended to limit the
scope of the claims.
[0056] Background and Prior Art
[0057] FIG. 1 shows a typical prior art backlight 100 as used with
transmissive or
transflective display 110. The backlight 100 is placed behind the display 110
from a viewer's
perspective. Although, other backlight designs and principles exist, here the
description shall
be given using the example of an edge-lit lightguide-based backlight. The
backlight 100
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comprises a lightguide 140, which is typically made from a transparent polymer
such as
PMMA, PC or glass.
[0058] Lightguide 140 is similar in size with respect to length and width
dimensions to
display 110 and serves the purpose of spreading the light uniformly across the
display 110.
The lightguide 140 has features that may be in the bulk or on the surface.
Such features in the
surface may be dimples or prisms, in the bulk they may be randomly or
regularly distributed
alternate materials. The features are designed as to cause the light 150 to
leave the lightguide
140 in a defined direction and with a uniform intensity distribution as shown
in FIG. 1.
[0059] The features may send the light 150 directly to display 110 or first to
a reflector
behind the lightguide 140 from observer's 160 perspective, from where the
light 150 is
reflected towards the display 110. Lightguide 140 may incorporate other
optical functions,
such as diffusion or light shaping and directing, or these functions may be
added in separate
components (usually sheets of films) placed into the immediate vicinity of the
lightguide 140
or adhered to it.
[0060] Illuminators 130 are placed along one or more edges of the lightguide
140. In one
embodiment, illuminators 130 can be side firing LEDs. It is important that the
light 150 is
effectively coupled into the lightguide 140 without excessive waste. This is
achieved by the
design of the interface between illuminator 130 and lightguide 140 as well as
the design of
the light sources such that the emitted light 150 exits the light source in a
useful range of
angles. Typically, a flexible printed circuit 120 provides the electrical
current to operate the
illuminators, which may be arranged electrically in series, parallel, or in
parallel groups of
smaller series.
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[0061] In one embodiment, the LEDs comprising the illuminators 130 may be
selected to
emit white, red, green, blue, or infrared (IR) light or any combination
thereof One backlight
may have LEDs with multiple, different emission wavelengths.
[0062] FIG. 2 shows a typical front light 200 as used in conjunction with
reflective
displays, such as reflective display 210. The elements of front light 200 are
similar to that of
backlight 100 and include flexible printed circuit board 220, illuminators
230, and lightguide
240. However, front light 200 is placed in front of the display 210 from an
observer's 260
perspective, as compared to backlight 100. Additional requirements for front
light 200 are
that the light 150 must be exclusively directed towards the display 210 and
that the front light
200 must allow for a clear, sharp, and accurate color display image.
[0063] FIG. 3 shows a liquid crystal display 300 with backlight 310 according
to U.S.
Patent No. 9,190,004. The backlight 310 is arranged behind the display 300.
The backlight
310 comprises LED light source 320, a lightguide plate 340, a reflector 350
behind the
lightguide plate 340, and optional light shaping films 330, located between
the lightguide
plate 340 and the display 300. The display 300 comprises a front polarizer
360, an LCD cell
370, and a rear polarizer 380. The LCD cell 370 also comprises a liquid
crystal layer 390
interspersed between a front substrate 392 and a rear substrate 394.
[0064] U.S. Patent No. 9,190,004 teaches that the LED light source 320 may
emit visible
and/or IR light and that the front polarizer 360 and rear polarizer 380 must
be able to polarize
both visible and infrared light in order for the display 300 to have contrast
in the visible and
infrared region of the electromagnetic spectrum. Typical liquid crystal
display polarizers 360
and 380 work only in the visible range, where they absorb one polarization
while
transmitting the other polarization. Moreover, adding anisotropic IR absorbing
chromophores
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to visible light polarizers would be required, and it is not disclosed how
this can be done nor
that such additional chromophores would reduce the visible light transmission
and thus lead
to a much dimmer display.
[0065] U.S. Patent No. 9,191,004 teaches that certain reflective polarizers
can be used such
as wire grid polarizer sheets, birefringent polarizer sheets, or cholesteric
liquid crystal
polarizer sheets, which work both in the visible and IR region. The
disadvantage of such
polarizers is that they reflect, not absorb the unwanted polarization. While
that can be dealt
with in the rear polarizer 380, it is undesirable in front polarizer 360 as it
gives the display a
mirror-like appearance. Bright image areas in such a display appear bright,
while dark image
areas appear like a metallic mirror. An observer will see both the display
image and a
reflected scene superimposed onto each other.
[0066] FIG. 4 shows prior art in accordance with U.S. Patent No. 9,229,268,
which is an
improvement of U.S. Patent No. 9,190,004 as it eliminates the undesirable
mirrorlike
appearance.
[0067] The backlight 310 again comprises LED light source 320, a lightguide
plate 340, a
reflector 350 behind the lightguide plate 340, and optional light shaping
films 330, located
between the lightguide plate 340 and the display 400.
[0068] Backlight 310 is placed behind liquid crystal display 400 comprising
the LCD cell
370, a sequential stack of two front polarizers comprising visible light front
polarizer 460 and
IR light front polarizer 465, and a sequential stack of two rear polarizers
comprising visible
light rear polarizer 480 and IR light rear polarizer 485. The LCD cell 370
also comprises a
liquid crystal layer 390 interspersed between a front substrate 392 and a rear
substrate 394.
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[0069] In one embodiment, one each of the front polarizers 460 and 465, and
the rear
polarizers 480 and 485 is a reflective type, such as a wire grid polarizer
sheet, a birefringent
polarizer sheet, or a cholesteric liquid crystal polarizer sheet, which work
both in the visible
and IR region. The other one of the front polarizers 460 and 465 and the rear
polarizers 480
and 485 is a standard liquid crystal display polarizer sheet, which work only
in the visible
range by absorbing one polarization, while transmitting the other
polarization. This works
because the standard absorbing polarizer sheets are transparent for both
polarization states in
the IR region.
[0070] Hence, in the infrared region the display 400 still is a mirror-like
display, while in
the visible region the display looks as expected with black and bright image
areas. A human
observer will not see the reflected infrared light, while infrared equipment
used to view the
display has to be arranged and designed such that the reflected unwanted
polarization is not
detrimental to the image quality or function of the IR system, such as by
using an optical
pattern or character recognition system. Using the two front polarizers 460
and 465 and the
two rear polarizers 480 and 485, rather than one of each, adds two costly
elements to the
display 400 and since both types of polarizers are not 100% transmissive in
the visible region
of the electromagnetic spectrum, the brightness of the display is diminished
by the second
polarizer, which is unnecessary for visible light.
[0071] In addition, neither U.S. Patent No. 9,190,004 nor U.S. Patent No.
9,229,268 allow
the use of a reflective display, which cannot be operated with a backlight.
Neither patent
teaches the use of a front light, but if the proposed structure were
illuminated with a front
light and backed with the necessary reflector, the display would be very dim
as the light
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would travel through a sequence of eight polarizer layers, each absorbing a
significant
portion of the light.
[0072] Thus, this disclosure describes systems and methods of IR readable
transmissive
and reflective displays without a mirror appearance and without the unwanted
dimming of
the display due to sequential stacks of polarizers.
[0073] Infrared Transmissive LCD Display 500A and B with a Reflective Rear
Polarizer 510 and a Non-Polarized Infrared Backlight 505A and B
[0074] FIG. 5A shows one example embodiment of this invention consisting of an
infrared
transmissive type LCD display 500A with backlight 505A. As an infrared
transmissive type
display, LCD display 500A is generally transmissive to infrared light and
reflective of
ambient visible light.
[0075] Backlight 505A comprises light sources such as LEDs 515, emitting
visible and
infrared light, a lightguide 520, an absorber 525 placed behind lightguide 520
from an
observer's 530 vantage point, and in one embodiment light directing and
diffusing films 535
in front of the lightguide 520 and behind LCD display 500A. Optional light
directing and
diffusing films 535 may include optical films such as retardation films,
compensation films,
and other light management films that optimize the performance of the device.
[0076] LCD display 500A further comprises an LCD cell 540 between a visible
light front
polarizer 545 and IR capable reflective rear polarizer 510. LCD cell 540
comprises a liquid
crystal layer 550 interspersed between a front substrate 555 and a rear
substrate 560.
[0077] The front polarizer 545 is of an absorptive type, which absorbs visible
light of the
unwanted polarization, while transmitting visible light of the desired
polarization as well as
all infrared light irrespective of polarization. The rear polarizer 510 is a
reflective type such
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as a wire grid polarizer sheet, a birefringent polarizer sheet or a
cholesteric liquid crystal
polarizer sheet, which work both in the visible and IR regions.
[0078] An infrared sensitive image capture or recording device such as camera
570 is
directed towards LCD display 500A. In one embodiment, camera 570 comprises an
infrared
camera. Camera 570 comprises a lens 575 to focus the display image onto the
sensor
element (not shown) inside the camera 570. It also comprises an IR analyzer
580 to avoid
glare for reflective surfaces like the polarizers used in photographic
equipment to reduce
glare, only with its function optimized for IR wavelengths.
[0079] Also directed towards LCD display 500A is observer 530 viewing LCD
display
500A via reflection of light from visible environmental light source 590.
Environmental
light source 590 may be diffuse daylight, direct sunlight, room light, light
from a dedicated
illumination source or similar.
[0080] In FIGS. 5 ¨ 11, unpolarized light 592 is shown as arrows with a solid
line, a dotted
line illustrates a first polarization 594, such as linear s-polarization or
circular 1-polarization,
and a dashed line illustrates the second orthogonal to the first polarization
596, such as linear
p-polarization or circular r-polarization.
[0081] In visible light observation of LCD display 500A, unpolarized light 592
from the
environmental light source 590 enters LCD display 500A. The portion of the
unpolarized
light 592 with the undesired polarization is absorbed in front polarizer 545.
Light of the
desired polarization is transmitted through front polarizer 545 into LCD
display 500A, where
in the liquid crystal layer 550 the light either retains its polarization 596
or has its
polarization morphed or changed into the orthogonal polarization 594,
depending on the state
of the liquid crystal layer 550.
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[0082] Unchanged light is transmitted through reflective rear polarizer 510
and backlight
505A until it gets absorbed in absorber 525. The corresponding display area
looks black or
dark to the observer 530. Light with a changed polarization state is reflected
by the rear
polarizer 510 and changed back to its original polarization state in the
liquid crystal layer
550, and therefore has the correct polarization state to pass the front
polarizer 545 and then
travels to the observer 530. The corresponding area of LCD display 500A
appears bright to
the observer 530. For visible observation of light from environmental light
source 590,
backlight 505A is not required, however, absorber 525 must be provided.
[0083] For infrared observation, the IR LEDs 515 are activated. Unpolarized IR
light from
IR LEDs 515 spreads through lightguide 520 and illuminates LCD display 500A
uniformly
from behind. After passing the light shaping and diffusing films 535, the
light is polarized in
reflective rear polarizer 510 as only one polarization state is transmitted,
while the other
polarization state is reflected. The reflected portion of the light is
returned into lightguide 520
and may get absorbed in absorber 525.
[0084] The polarized light travels through liquid crystal layer 550 and
depending on the
alignment of the liquid crystals in liquid crystal layer 550, the light
retains its polarization or
changes to another polarization state. 'Bright' and 'Dark' areas of the image
emit the same
amount of light but with different polarization states. One of these states
can pass IR analyzer
580 of camera 570 while the other polarization is rejected.
[0085] Therefore, bright and dark areas are projected by lens 575 onto the
image sensor
inside camera 570, corresponding to the polarization state emitting from the
respective areas
of the display. If desired, the contrast of the image can electronically be
inverted before
image analysis or before displaying the image on LCD display 500A. Because
only one
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polarizer is used on either side of LCD display 500A there is no additional
cost, thickness,
and undue dimming of the brightness of LCD display 500A. Because the front
polarizer 545
is an absorbing type, there is no mirror-like image display surface.
[0086] In an alternative embodiment, backlight 505A can be used in conjunction
with
visible light from LEDs 515. In this embodiment, the visible light image using
backlight
505A has inverted contrast. Such a display arrangement may use a visible light
sensor (not
shown) and activate backlight 505A while simultaneously electronically
reversing the
contrast of the image, so that the observer sees the proper contrast (twice
inverted).
[0087] It should be clear to those skilled in the art that LCD display 500A
can be operated
with the transmission axes of front polarizer 545 and rear polarizer 510
either orthogonal
(crossed) or parallel and with the liquid crystal layer 550 being arranged to
retain the
polarization either when powered or not or either in one or another stable
state. This leads to
several possible alternative embodiments often referred to as normally white
and normally
black with direct or inverse contrast.
[0088] FIG. 5B shows an alternative embodiment of LCD display 500A shown in
FIG. 5A.
In FIG. 5B, backlight 505B has reflector 527 as the element furthest from
observer 530,
instead of absorber 525 in backlight 505A. In this embodiment, LCD display
500B has
visibly opaque (black) layer 526 added to LCD display 500B near backlight
505B. Visibly
opaque (black) layer 526 is transparent for infrared light but absorbs visible
light. Visibly
opaque (black) layer 526 can be printed with special dyes such as Epolight
dyes, or it can be
formed by a visible light absorptive polarizer with its transmission axis
orthogonal to
(crossed) that of the reflective rear polarizer 510. In other embodiments,
visibly opaque
(black) layer 526 can comprise other colors such as a blue opaque layer that
transmits IR
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which leads to an embodiment of LCD display 500B with a blue and white
contrast rather
than a black and white contrast.
[0089] LCD display 500B has the advantage of reduced parallax shadow in
visible light
operation as absorber 525 is closer to liquid crystal layer 550. In infrared
operation, the
reflected polarization of the infrared light at the rear polarizer 510 is
recycled in the backlight
505B. This increases the efficiency of backlight 505B.
[0090] Infrared Transmissive LCD Display 600 with Absorptive Visible Light
Rear
Polarizer 610 and Polarized Infrared Backlight 605
[0091] FIG. 6 shows another embodiment of the invention. Infrared transmissive
type
LCD display 600 is similar to LCD display 500A except reflective rear
polarizer 510 has
been replaced by absorptive visible light rear polarizer 610. Backlight 605 is
similar to
backlight 505B except the IR polarizer 612 is added between the IR LEDs 515
and lightguide
620. In this embodiment, lightguide 620 is polarization conserving.
[0092] For visible light observation, unpolarized light 592 from environmental
light source
590 enters LCD display 600. The portion of the unpolarized light 592 with the
undesired
polarization is absorbed in front polarizer 645. Light of the desired
polarization is transmitted
into LCD display 600, where, in the liquid crystal layer 550, the light either
retains its
polarization 596 or has its polarization morphed or changed into orthogonal
polarization 594,
depending on the alignment of the liquid crystals in liquid crystal layer 550.
Unchanged light
is absorbed in rear polarizer 610. The corresponding area of LCD display 600
looks black to
observer 530.
[0093] Light with a changed polarization state is transmitted through rear
polarizer 610 and
backlight 605 and is reflected by reflector 527. The reflected light passes
through rear
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polarizer 610 and is changed back to its original polarization state in the
liquid crystal layer
550 and therefore has the correct polarization state to pass through front
polarizer 545 and
then travel to the observer 530. The corresponding area of LCD display 600
appears bright to
observer 530. For visible observation with light from environment light source
590, backlight
605 is not required, but reflector 527 must be provided.
[0094] In an alternative embodiment, backlight 605 can be used with optional
visible light
sources (not shown) if there is insufficient environmental light. In this
case, unpolarized light
592 exiting lightguide 620 is polarized by rear polarizer 610 and remains
unchanged or has
its polarization state changed in the liquid crystal layer 550, depending on
the orientation of
the liquid crystals. Light with a changed polarization state passes through
front polarizer 545
and reaches the observer 530. The corresponding image area of LCD display 600
looks
bright to observer 530. Light with an unchanged polarization state gets
absorbed in the front
polarizer 545. The corresponding image area of LCD display 600 looks dark to
observer 530.
This arrangement does not cause a contrast inversion.
[0095] For infrared observation, IR LEDs 515 are activated. Unpolarized IR
light from IR
LEDs 515 is polarized with IR polarizer 612 before entering lightguide 620.
Polarized IR
light spreads through lightguide 620 and illuminates LCD display 600 uniformly
from
behind.
[0096] After passing the optional light shaping and diffusing films 535, the
light passes
rear polarizer 610 unchanged as this polarizer type appears transparent to IR
light. The
polarized light travels through liquid crystal layer 550 where, depending on
the alignment of
the liquid crystals in liquid crystal layer 550, the light retains its
polarization or changes to
another polarization state. 'Bright' and 'Dark' areas of the image emit the
same amount of
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light but with different polarization states. One of these states can pass IR
analyzer 580 of
camera 570, while the other polarization is rejected.
[0097] Therefore, bright and dark areas are projected by lens 575 onto the
image sensor
inside camera 570, corresponding to the polarization state emitting from the
respective areas
of LCD display 600. If desired, the contrast of the image can electronically
be inverted
before image analysis or before displaying the image on LCD display 600. Since
only one
polarizer is used on either side of LCD display 600 there is no additional
cost, thickness, and
undue dimming of the display brightness. Because front polarizer 545 is an
absorbing type,
there is no mirror-like image display surface for LCD display 600.
[0098] Those skilled in the art will appreciate that alternative embodiments
have
equivalents with parallel and crossed polarizers, and different liquid crystal
director
configurations, some of which may be more or less advantageous.
[0099] Reflective LCD Display 700 with Reflective Rear Polarizer 510 and with
non-
polarized IR Front light 705
[0100] FIG. 7 illustrates another embodiment of the invention, based on a
reflective type
LCD display 700. A front light 705, comprising LED illuminators 515 and a
light guide 720,
is placed between the front polarizer 545 and LCD cell 740. The front
polarizer 545 is
absorptive, which works for visible light, but appears transparent to infrared
light. Rear
polarizer 510 is reflective and works for both visible and infrared light.
Located behind the
rear polarizer 510 is an absorber 525.
[0101] For visible light observation, unpolarized light 592 from environmental
light source
590 enters LCD display 700. The portion of the light with the undesired
polarization is
absorbed in front polarizer 545. Light of the desired polarization 596 is
transmitted through
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front polarizer 545 and front lightguide 720 and into LCD display 700, where,
in the liquid
crystal layer 750, it either retains its polarization or has its polarization
morphed or changed
into the orthogonal polarization 594, depending on the alignment of the liquid
crystals in
liquid crystal layer 750.
[0102] Unchanged light 596 is transmitted through reflective rear polarizer
510 and gets
absorbed in absorber 525. The corresponding area of LCD display 700 looks dark
to observer
530. The observer 530 'sees' the black absorber 525. Light with a changed
polarization state
594 is reflected by rear polarizer 510, changed back to its original
polarization state 596 in
liquid crystal layer 750, and therefore has the correct polarization state to
pass through front
polarizer 545 and then travel to observer 530. The corresponding area of LCD
display 700
appears bright to observer 530. For visible observation with light from
environment light
source 590, front light 705 is not required.
[0103] In an alternative embodiment, front light 705 can be used with optional
visible light
sources (not shown) and a visible light polarizer (not shown) between the
visible light
sources and lightguide 720 if there is insufficient light from environmental
light source 590.
In this case, polarized light exiting the lightguide 720 takes the same path
as environmental
light after passing through front polarizer 745.
[0104] In another alternative embodiment, front light 705 can be positioned in
front of
front polarizer 745. In this case, the visible light polarizer (not shown)
between visible light
source (not shown) and lightguide 720 is not necessary.
[0105] For infrared observation, IR LEDs 515 are activated. Unpolarized IR
light from the
light source 590 spreads through lightguide 720 and illuminates LCD display
700 uniformly
from the front. Because front polarizer 545 is transparent to IR light, in a
modification of this
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embodiment, front light 705 can also be in front of front polarizer 545 from
the vantage point
of observer 530. The light exiting lightguide 720 towards LCD display 700
travels
unchanged through liquid crystal layer 750 to rear polarizer 510, where light
of one
polarization is reflected into LCD display 700 while light of the other
polarization is
transmitted through reflective rear polarizer 510 and is absorbed in absorber
525.
[0106] The polarized light reflected into LCD display 700 travels through the
liquid crystal
layer 750 where, depending on the alignment of the liquid crystal in layer
750, the light
retains its polarization or changes to another polarization state. The front
light 705 and front
polarizer 545 are transparent to IR light. 'Bright' and 'Dark' areas of the
image emit the
same amount of light but with different polarization states. One of these
states can pass the
IR analyzer 580 of camera 570 while the other polarization is rejected.
[0107] Therefore, bright and dark areas are projected by lens 575 onto the
image sensor
inside camera 570, corresponding to the polarization state emitting from the
respective areas
of the display 700. If desired, the contrast of the image can electronically
be inverted before
image analysis or before displaying the image on LCD display 700. Since only
one polarizer
is used on either side of LCD display 600 there is no additional cost,
thickness, and undue
dimming of the display brightness. Because the front polarizer 545 is an
absorbing type, there
is no mirror-like image display surface for LCD display 600.
[0108] Reflective LCD Display 800 with Reflective Rear Polarizer 510 and
Polarized
Infrared Front Light 805
[0109] FIG. 8 illustrates another embodiment of the invention. Reflective type
LCD
display 800 is similar to LCD display 700 except IR polarizer 812 is placed
between the IR
LEDs 515 and front lightguide 820. Front lightguide 820 must be polarization
maintaining.
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[0110] The light path and functional principle for visible light observation
for LCD display
800 is the same as for LCD display 700. For infrared observation, IR LEDs 515
are
activated. Unpolarized IR light from IR LEDs 515 is polarized with IR
polarizers 812 at the
light source. Polarized IR light spreads through the polarization maintaining
lightguide 820
and illuminates the LCD display 800 uniformly from the front. In another
modification of
this embodiment, because the front polarizer 545 is transparent to IR light,
the front light 805
can also be in front of the front polarizer 545.
[0111] The polarized light exiting lightguide 820 into LCD display 800 travels
unchanged
to liquid crystal layer 750 where, depending on the orientation of the liquid
crystal in liquid
crystal layer 750, the light retains its polarization or changes to another
polarization state.
Unchanged light 596 is transmitted through reflective rear polarizer 510 and
is absorbed in
absorber 525. The corresponding area of LCD display 800 is imaged via lens 575
as a black
area onto the sensor inside camera 570 since no light is traveling to the
camera.
[0112] Light with a changed polarization state 594 is reflected by rear
polarizer 510 and is
changed back to its original polarization state 596 upon passing liquid
crystal layer 750 a
second time. Front polarizer 545 is transparent to IR light, so the light
continues to camera
570. The corresponding area of LCD display 800 is imaged via lens 575 as a
bright area onto
the sensor inside camera 570. In this configuration, IR analyzer 580 is not
required. If the
camera system has an antiglare IR polarizer 580, the polarization directions
of LCD display
800 must be configured such that the light 596 traveling to camera 570 is
substantially
vertically polarized. This ensures the light 596 can pass the antiglare IR
polarizer 580, which
blocks horizontally polarized light.
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[0113] Digital license plate with reflective LCD, Front Light and combined
infrared
and visible light illumination
[0114] FIG. 9 illustrates display system 900, which in one embodiment is the
display
system of a digital license plate that requires visible readability and IR
optical pattern or
character recognition. In this example, for display system 900, LCD display
800 is placed
into housing 905. However, in other embodiments, LCD display 500, LCD display
600, or
LCD display 700 could also be used in place of LCD display 800 in display
system 900. The
housing 905 comprises a front lens 950, an IR light sensor 955 sensitive only
to IR light, and
a daylight sensor 960 sensitive only to visible light wavelengths. In
addition, in one
embodiment, additional optional visible light LEDs 915 may be added.
[0115] Digital license plates (not shown) must be readable by automated
license plate
recognition (ALPR) camera system 975. ALRP camera system 975 comprises one or
more
ALRP cameras 970, working with visible and IR light and infrared light
illuminators 980,
next to the ALRP camera 970. ALRP camera system 975 is optimized to read
retroreflective
license plates or license plates with diffuse, Lambertian reflectance and is
necessarily placed
above or to the side of the roadway or on another vehicle.
[0116] The illumination from IR illuminator 980 is coaxial with ALPR camera
970. Such
light, however, is reflected by a digital license plate substantially away
from ALPR camera
970, rather than back towards ALPR camera 970. This necessitates the use of
internal IR
illumination of the license plate.
[0117] Display system 900 has suitable electronic circuits (as shown in FIG.
10) that
activate IR LEDs 515 when IR light sensor 955 senses a rapid change in IR
intensity, which
occurs, for example, if LCD display 800 is being flashed with ALPR camera 970
or if a
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vehicle drives into an IR flood illumination zone. Such electronic circuits
may be based on a
microcontroller unit and firmware determining when to turn on IR LEDs 515.
[0118] Figure 10 shows display system 900 in the form of block diagram 1000
consisting
of microcontroller unit (MCU) 1040 connected to battery 1010, day light sensor
960, IR
sensor 955, optional peripherals 1050, LCD display 800, and illuminator unit
1070 consisting
of visible light LEDs 915, second light source 1082, IR LEDs 515, and fourth
light source
1086. Illuminator unit 1070 is used to illuminate display 800. In alternative
embodiments,
additional light sources may also be used.
[0119] Alternatively, for lower power consumption and faster response, such
circuits may
connect the IR light sensor 955 to an operational amplifier, which drives a
current source for
IR LEDs 515, causing IR LEDs 515 to flash back in sync with being flashed by
IR light
without the digital license plate and its microcontroller system having to
wake up from a low
power state.
[0120] Figure 11 shows such an alternative layout of display system 900 in the
form of
block diagram 1100 consisting of MCU 1040 connected only to battery 1010,
optional
peripherals 1050, and LCD display 800. Light sensor circuit 1110 is connected
directly to the
battery and controls illuminator unit 1070, visible light LEDs 915, and IR
LEDs 515.
Illuminator unit 1070 is used to illuminate display 800.
[0121] Figures 12 shows an example of light sensor circuit 1110 that can be
used to detect
light and drive the IR LEDs 515 without involvement of MCU 1040, consisting of
IR sensing
board 1210 and LED driving board 1220.
[0122] Turning back to FIG. 9, the internal IR illumination of display system
900
represents an active response to interrogation, directed towards ALPR camera
970 resulting
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in a brighter image, lower signal-to-noise ratio and hence a better accuracy
of the optical
pattern or character recognition. The IR wavelength of the interrogating
system and the IR
wavelength of the active response are independent. The IR wavelength of
internal IR LEDs
515 illuminating LCD display 800 can be chosen to achieve the best possible
contrast and
accuracy in the IR image capture and recording system.
[0123] The reflective LCD display 800 may be a bi-stable or multi-stable LCD
due to the
low power requirements of such displays compared to displays requiring
constant updating.
In one embodiment, the liquid crystal display maintains a stable visible image
without being
refreshed more than once per second. One such bistable LCD type may be a
memory-in-pixel
LCD, another may be a bistable nematic LCD known as Binem, or a bistable
nematic display
known as ZBD.
[0124] The LCD display 800 can work with a reflective rear polarizer 510, such
as a
multilayer polymer stack available from 3MTm known as DBEF, a wire grid
polarizer such as
WGF from Nagase, or similar. Such reflective polarizers have usable contrast
from about
380nm to greater than 850nm.
[0125] Front lightguide 820 may be located on top or below front polarizer
845. It is
illuminated from the edge with optional white light LEDs 915 for night
visibility, controlled
by daylight sensor 960, and with a plurality of IR LEDs 515 selected for a
desired
wavelength or multiple desired wavelengths depending on the requirements of
the location
where such a license plate is issued. Automated license plate recognition
systems operate at
specific infrared wavelengths, such as 740nm, 850nm, 940nm and others. For
example, IR
LED's may comprise several 740 nm and several 850nm LEDs if that matches the
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requirement. One of skill in the art would understand that other combinations
are possible as
well.
[0126] Polarizer 812 may be a dye type polarizer with dye selected for
infrared operation
and it is not required to have good transmission or polarization efficiency in
the visible
spectrum as no visible light is required to pass through it. Another suitable
type of polarizer
may be a wire grid type polarizer or multi-layer stack polarizer as such
polarizers are simpler
and easier to produce at a lower cost than wire grip polarizers for the
visible range.
[0127] Polarization preserving lightguide 820 may be made from transparent
polymers,
glass, or a combination of different transparent materials and may be coated
with materials of
a different refractive index.
[0128] Also shown in FIG. 9 is the light path 990 for observer 530 in night
mode. If the
daylight sensor 960 detects a dark environment, it may activate the visible
light LEDs 915.
Unpolarized light from visible light LEDs 915 travels through the lightguide
820 from where
it is directed uniformly towards LCD display 800. Because the light is
unpolarized it passes
through the display unchanged.
[0129] Part of the light is reflected at rear polarizer 510, while light of
the undesirable
polarization passes through rear polarizer 510 and is absorbed by absorber
525. The reflected
polarized light will either remain unchanged or its polarization will be
changed by the liquid
crystal layer 850, depending on the orientation of the liquid crystals in
liquid crystal layer
750.
[0130] Light with an unchanged polarization state will be absorbed by front
polarizer 545.
The corresponding image areas of LCD display 800 appear dark to observer 530.
Light with
a change in polarization state passes front polarizer 545 and reaches observer
530.
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Corresponding display areas of LCD display 800 appear bright. In an
alternative
embodiment, front light 805 can also be placed in front of front polarizer
845. In this case
visible light exiting lightguide 820 will first be polarized by the absorptive
front polarizer
845 before passing to the display in an analogous fashion. In yet another
alternative
embodiment, a visible light polarizer can be placed between visible light LEDs
915 and
lightguide 820.
[0131] While the invention has been specifically described in connection with
certain
specific embodiments thereof, it is to be understood that this is by way of
illustration and not
of limitation. Reasonable variations and modifications are possible within the
scope of the
foregoing disclosure and drawings without departing from the spirit of the
invention.
34
